Did Jesus believe in life after death all of his earthly life? Was His knowledge gained from the Torah, or was it revealed specifically to Him by God?
Christians believe that Jesus was God in the flesh and thus the author of the Torah (the five books of the Old Testament recorded by Moses). It can be shown several ways, but the clearest is found in John. "In the beginning was the Word, and the Word was with God, and the Word was God. He was in the beginning with God. All things were made through Him, and without Him nothing was made that was made" (John 1:1-3). Then a few verses later we read, "And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth" (John 1:14). Paul talks of the same fact. Speaking of Christ, Paul said, "For in Him dwells all the fullness of the Godhead bodily" (Colossians 2:9).
Throughout Jesus' ministry, he taught people about eternal life. For example, "And as Moses lifted up the serpent in the wilderness, even so must the Son of Man be lifted up, that whoever believes in Him should not perish but have eternal life" (John 3:14-15). Such a life cannot be referring to our earthly life since we all die. The existence of an eternal life implies a life that continues on after death. Jesus also taught people that there would be a resurrection and judgment. "Most assuredly, I say to you, he who hears My word and believes in Him who sent Me has everlasting life, and shall not come into judgment, but has passed from death into life. Most assuredly, I say to you, the hour is coming, and now is, when the dead will hear the voice of the Son of God; and those who hear will live. For as the Father has life in Himself, so He has granted the Son to have life in Himself, and has given Him authority to execute judgment also, because He is the Son of Man. Do not marvel at this; for the hour is coming in which all who are in the graves will hear His voice and come forth--those who have done good, to the resurrection of life, and those who have done evil, to the resurrection of condemnation" (John 5:24-29).
This should not have been a new teaching to the Jews. Jesus scolded his audience on several occasions for not understanding the information found in the Old Testament. "You search the Scriptures, for in them you think you have eternal life; and these are they which testify of Me. But you are not willing to come to Me that you may have life" (John 5:39-40). "The same day the Sadducees, who say there is no resurrection, came to Him and asked Him, saying: "Teacher, Moses said that if a man dies, having no children, his brother shall marry his wife and raise up offspring for his brother. Now there were with us seven brothers. The first died after he had married, and having no offspring, left his wife to his brother. Likewise the second also, and the third, even to the seventh. Last of all the woman died also. Therefore, in the resurrection, whose wife of the seven will she be? For they all had her." Jesus answered and said to them, "You are mistaken, not knowing the Scriptures nor the power of God. For in the resurrection they neither marry nor are given in marriage, but are like angels of God in heaven. But concerning the resurrection of the dead, have you not read what was spoken to you by God, saying, 'I am the God of Abraham, the God of Isaac, and the God of Jacob'? God is not the God of the dead, but of the living." And when the multitudes heard this, they were astonished at His teaching" (Matthew 22:23-33).
We also know that those Jesus taught understood that there was life after death. After Lazarus died, Jesus went and spoke to his sister Martha. "Then Martha said to Jesus, "Lord, if You had been here, my brother would not have died. But even now I know that whatever You ask of God, God will give You." Jesus said to her, "Your brother will rise again." Martha said to Him, "I know that he will rise again in the resurrection at the last day." Jesus said to her, "I am the resurrection and the life. He who believes in Me, though he may die, he shall live. And whoever lives and believes in Me shall never die. Do you believe this?" She said to Him, "Yes, Lord, I believe that You are the Christ, the Son of God, who is to come into the world."" (John 11:21-27).